And improve its wording.

The flow of the text implied that the OpenXR-SDK is a requirement before
compiling Monado.
This moves the OpenXR-SDK section to "Running OpenXR Applications".

Credit for the style to https://stackoverflow.com/a/36658757

On tegra jp45 you get this error without it:

Running with trace output on.
CMake Error: The source directory "" does not exist.

* Update Index page
  * Update current state of Monado
  * Update support matrix
* Improve getting started documentation
  * Show explanation of debian/ubuntu packages more prominently
  * Improve service enabled vs service disabled build explanations
* Improve application developing examples
* Update Positional Tracking
  * Add PSVR setup info
  * Improve camera calibration procedure steps explanations